Electron–ion recombination and photoionization of Fe XXI

Abstract

Results for electron–ion recombination and photoionization of ( Fe XXI + h ν ↔ Fe XXII + e ) , with emphasis in high-temperature region, are presented from ab initio unified method. The unified method, based on close coupling (CC) approximation and R-matrix method, (i) subsumes both the radiative recombination (RR) and dielectronic recombination (DR), (ii) enables self-consistent sets of photoionization and recombination cross sections from using an identical wavefunction for both the processes, and (iii) provides state-specific recombination rates of a large number of bound states. A large CC wavefunction expansion, which includes the ground and 28 core excitations of n = 2 and 3 complexes and span a wide energy range, has been used. Compared to Δ n = 2 – 2 , Δ n = 2 – 3 core excitations are found to introduce strong resonant structures and enhance the background photoionization cross sections ( σ PI ) in the high-energy region. These features along with prominent photoexcitation-of-core (PEC) resonances at n = 3 core thresholds have increased the unified total recombination rate coefficients ( α R ( T ) ) at temperatures T > 10 6 K , region of maximum abundance of the ion in collisional equilibrium, by a factor of 1.6 over previous calculations. State-specific recombination rate coefficients α R ( nLS ) , which include both the RR and DR, are presented for the first time for 685 bound states with n ⩽ 10 and l ⩽ 9 . The unified total recombination rate with photoelectron energy α R ( E ) is presented and the role of low-energy near-threshold fine structure resonances is illustrated. The present results should provide a reasonably complete self-consistent set of recombination rates and photoionization cross sections for astrophysical modelings of high-temperature plasmas from optical to far-ultraviolet wavelength regions.